# Primary & secondary inductance?

Discussion in 'General Electronics Chat' started by electronice123, Oct 1, 2014.

1. ### electronice123 Thread Starter Senior Member

Oct 10, 2008
304
0
In transformer design what role do the primary and secondary inductances play, and why are they designed to have a certain inductance?

The reason why I am asking is I keep seeing transformers used for different purposes that all have different inductances. I know that with an ideal transformer the source sees the load as if were powering it directly. So why do different transformers have different inductance values if they are not seen by the source?

What is it about the design that requires the primary and secondary coils to have specific inductances?

Last edited: Oct 1, 2014
2. ### wayneh Expert

Sep 9, 2010
12,389
3,245
The inductance ratio relates to the turns and voltage ratio. I forget the math but I think it's L^2 ratio is inverse to voltage ratio.

3. ### electronice123 Thread Starter Senior Member

Oct 10, 2008
304
0
Yes I know, but my question is. Why do transformers with the same turns ratio have different inductances-what determines the inductance needed?

Is it just whatever # of primary turns are required to prevent saturation?

4. ### MrCarlos Active Member

Jan 2, 2010
400
134
Hello electronice123

In transformer design what role do the primary and secondary inductances play, and why do they have to be a certain value?
The inductances of the primary and secondary should have a certain value in tuned circuits.

You Say:
The reason why I am asking is I keep seeing transformers used for different purposes that all have different inductances.
Course, depending on the purpose it is the value of inductance required.
It's not the same a transformer tuned to 1 MHz for another tuned to 100KHz.

I know that with an ideal transformer the source sees the load as if were powering it directly. So why do different transformers have different inductance values if they are not seen by the source?
I do not understand the meaning of the question.

What is it about the design that requires the primary and secondary coils to have specific inductances?
Primary and Secondary have a specific inductance because only this way resonate at the tuning frequency.

Why do transformers with the same turns ratio have different inductances-what determines the inductance needed?
Yes. Many transformers with the same turns ratio have different inductance value.
There are several things that determine the value of the inductance needed.
The ones I remember are:
The geometric shape of the coil.
The type of core used.
The thickness of the wires.
The space between turns.
And of course: the ratio of turns.
The figure of merit Q.
Etc. Etc.

I hope I've helped You with these wordy

5. ### crutschow Expert

Mar 14, 2008
13,501
3,375
The primary inductance of an AC transformer determines the magnetizing current. Normally the inductance is made sufficiently high so that the magnetizing current is small (ideally zero of course). The number of winding turns are selected to give a reasonable magnetizing current while keeping the peak flux density below the saturation point of the core material. Since the inductive reactance is proportional to frequency, the lower the primary frequency, the larger the inductance needed for a given magnetizing current.

This inductance is seen by the source as part of the magnetizing current but it is not seen by the load currents. That is because the flux caused by the primary load current is cancelled by the current flowing in the opposite direction due to the secondary load(s).

Note that this means a transformer core can be saturated by a higher than rated voltage, which increases the core flux density, but not by the load current which does not affect the core flux value. (It's a common error to think that load current can saturate a transformer.)

The secondary inductance is the just the primary inductance reflected to the secondary. It's not a separate inductance.

All this means that a transformer used in a high frequency switching converter can be much smaller than a power line transformer, since the required inductance for the high frequency transformer is less, allowing a smaller core. That's why modern switcher wall-worts are much smaller than the old power line transformer versions.

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